• Use hyphens in compound words and compound modifiers such as spot-check and follow-up message.

• Use en dashes as interval or span marks, as in 8–9 a.m.

• Use em dashes for dashing effect — to indicate abrupt shifts in thought or asides — as illustrated here.

En dashes and em dashes were so named because they're the length of the letters n and m. (In the old days, letterpress printers told the two apart by holding them beside their respective letters.) Don't worry if you've never heard of them. Many writers don't know en dashes exist, and many writers refer to em dashes simply as "dashes." No harm.

A few words of advice regarding em dashes.

If the sentence continues after the aside — as this one does — don't forget the second em dash. Also, to avoid confusing your reader, don't use more than one pair of em dashes in a single sentence. Finally, because em dashes are showy marks, use them sparingly.

Here's some more good news.

Unless you're a professional printer or a fastidious writer, forget about en dashes. Just use hyphens to mark your intervals, like this: 8-9 a.m. Most readers won't notice the difference, and if they do, they won't care.

Furthermore, some writers think the em dash produced by Microsoft Word is too long and ungainly, so they use the shorter en dash in its place.

Now for some bad news.

Of the three horizontal line marks, only the hyphen appears on your keyboard. The other two were omitted to reduce production costs and maintenance when the typing machine was invented in the late 19th century, so today we rely on computer technology to produce the desired mark. Here's how that works:

• Create hyphens by striking the underscore-hyphen key once, with no spaces before or after (xx-xx).

• Create en dashes by striking the hyphen key either once or twice — it doesn't matter which — with once space before and after (xx–xx), and then go back and delete the spaces (xx–xx).

• Create em dashes by striking the hyphen key twice, with no spaces before or after (xx—xx).

Some software programs include keyboard shortcuts for creating en dashes and em dashes. Good luck figuring those out.

One more complication — actually two.

Style manuals differ on whether to use spaces before and after em dashes (the dashing mark). The Associated Press Stylebook calls for spaces. The Chicago Manual of Style calls for no spaces. Whichever style you choose, be consistent throughout a document. When writing for publication, follow the publication's guidelines.

The final complication: There are actually two more dashes — 2-em dashes (used to indicate missing letters) and 3-em dashes (used to indicate missing words).

But I say enough is enough.

Stephen Wilbers offers training seminars in effective business writing. E-mail him at wilbe004@umn.edu. His website is www.wilbers.com.

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